Elon JOINS Twitter's Board of Directors. There's a new SHERIFF in town.

Elon Musk is joining the board of Twitter.

The company disclosed the news in a securities filing on Tuesday, one day after Mr. Musk revealed that he had bought a 9.2 percent stake in the social media giant, a purchase that appeared to make him its largest shareholder. The news sent Twitter’s shares skyrocketing more than 20 percent on Monday.

Twitter’s stock rose more than 6 percent in premarket trading on Tuesday after the news of Mr. Musk’s board appointment.

Mr. Musk had been quiet on his intentions for the purchase, which was worth about $2.9 billion before it was disclosed but has risen sharply in value since. He registered with a securities document called a 13G filing, indicating that he planned for the investment to be passive and that he did not intend to pursue a larger role in the company.

Twitter’s Tuesday announcement said that there were no “arrangements or understandings” between Mr. Musk and Twitter that led to his director role.

“It is theoretically possible” to have a passive stake in a company and join the board in a “passive monitoring capacity,” said Joshua Mitts, a professor of corporate law at Columbia Law School. But the presumption of acquiring a board seat would be to effectively affect change in the company, he said. “It doesn’t mean that you couldn’t try and convince the S.E.C. otherwise, but I think it would be an uphill battle,” Mr. Mitts said.